Interfacial tension and the behavior of microemulsions and macroemulsions of water and carbon dioxide with a branched hydrocarbon nonionic surfactant

Measurements of interfacial tensions for 2-ethyl-hexanol-(propylene oxide)∼4.5–(ethylene oxide)∼8 (2EH-PO4.5-EO8) at the planar water–CO2 interface and the surfactant distribution coefficient are utilized to explain microemulsion and macroemulsion phase behavior from 24 to 60 °C and 6.9 to 27.6 MPa. A CO2 captive bubble technique has been developed to measure the interfacial tension γ at a known surfactant concentration in the aqueous phase, with rapid equilibration at the water–CO2 interface. The surface pressure (γo − γ) decreases modestly with density at constant temperature as CO2 solvates the surfactant tails more effectively, but changes little with temperature at constant density. The area per surfactant at the CO2–water interface determined from the Gibbs adsorption equation decreases from 250 A2/molecule at 24 °C and 6.9 MPa, to 200 A2/molecule at 27.6 MPa. It was approximately twofold larger than that at the water–air interface, given the much smaller γo driving force for surfactant adsorption. For systems with added NaCl, γ decreases with salinity at low CO2 densities as the surfactant partitions from water towards the W–C interface. At high densities, salt drives the surfactant from the W–C interface to CO2 and raises γ. Compared with most hydrocarbon surfactants, this dual tail surfactant is unusually CO2-philic in that it partitions primarily into the CO2 phase versus the water phase at CO2 densities above 0.8 g/ml, and produces γ values below 1 mN/m. With this small γ, a middle phase microemulsion and a C/W microemulsion were formed at low temperatures and high CO2 densities, whereas macroemulsions were formed at other conditions.